1. Field of the Invention
This invention relates to an improvement in treatment of thiosulfate containing liquors by wet oxidation.
2. Description of the Prior Art
The treatment of coke oven gas (COG) scrubbing liquors by wet oxidation is common in Japan and is becoming known in the United States and Canada. Coke oven gas scrubbing liquors typically contain ammonium thiocyanate, ammonium thiosulfate, ammonia, elemental sulfur, and some organics. The liquors may also contain small amounts of hydrogen sulfide and hydrogen cyanide. The average analyses of seven different COG liquors were as follows:
NH.sub.4 SCN: 60 g/l PA1 (NH.sub.4).sub.2 S.sub.2 O.sub.3 : 65 g/l PA1 NH.sub.3 : 3 g/l PA1 Elemental S: 4 g/l PA1 Total COD: 85 g/l PA1 pH: 6-8
In conventional wet oxidation practice, the scrubbing liquor is pumped by a high pressure pump through a heat exchanger where the liquor is preheated, to a reactor where the oxidation takes place. Compressed air is mixed with the liquor before the liquor enters the heat exchanger, or alternately prior to entering the reactor. The heat of reaction released in the reactor raises the temperature from the inlet temperature at the reactor bottom to the outlet temperature at the top of the reactor. From the top of the reactor the oxidized liquor and exhaust vapors are conveyed back through the heat exchanger and out of the system through a pressure control valve. Conventional wet oxidation of COG scrubbing liquors is normally carried out at temperatures from 230.degree. C. to 300.degree. C. and at pressures from 500 pounds per square inch to about 2000 pounds per square inch (35 to 140 atmospheres).
Wet oxidation of COG scrubbing liquors is shown, for example in U.S. Pat. No. 3,855,390 to Matumoto et al., in which thiocyanates are oxidized to sulfates. Lime or limestone is added to the liquors prior to wet oxidation to avoid corrosion of the reactor by sulfuric acid. Free sulfur initially present in the scrubbing liquors is removed by filtration prior to wet oxidation.
In Japanese Patent Publication No. 1712, Jan. 17, 1977 of Nomoto et al., improved yield and purity of ammonium sulfate are claimed in wet oxidation of COG scrubbing liquors followed by gas-solid separation, evaporation and recycle of the small liquid stream.
Difficulties with conventional wet oxidation arise with the decomposition of thiosulfate which occurs during the oxidation process. When the waste scrubber liquor is mixed with air and heated to even moderate temperatures, 150.degree. C. or higher, the ammonium thiosulfate begins to oxidize by the following mechanism: EQU (NH.sub.4).sub.2 S.sub.2 O.sub.3 +H.sub.2 O+2O.sub.2 .fwdarw.(NH.sub.4).sub.2 SO.sub.4 +H.sub.2 SO.sub.4
One mole of sulfuric acid is formed from each mole of ammonium thiosulfate contained in the feed. After enough sulfuric acid is formed to neutralize the excess ammonia originally contained in the liquor the pH rapidly drops to 3.0 or lower and the thiosulfate remaining unoxidized rapidly decomposes by the following reaction to give sulfite and elemental sulfur: EQU S.sub.2 O.sub.3.sup.= +H.sup.+ .fwdarw.HSO.sub.3.sup.- +S
For a waste scrubber liquor with a composition the same as shown above as much as 7.5 to 11.2 grams per liter of sulfur could be formed at the beginning of the oxidation process.
Elemental sulfur is very resistant to oxidation, and even at oxidation temperatures of 230.degree. to 290.degree. C., all sulfur will not be oxidized from the waste liquor. Small amounts of sulfur in the feed, 4 grams per liter, can usually be handled in the wet oxidation systems. Some will oxidize and the remainder is carried out with the effluent as small suspended particles.
Larger amounts of elemental sulfur that result from decomposition of thiosulfate cause problems for wet oxidation reactors. One reactor with 20 g/l elemental sulfur in the feed was found to have completely filled up with liquid sulfur. In other cases the sulfur, a liquid at reactor temperatures, solidified on the surfaces of the heat exchangers causing blockage and loss of heat transfer efficiency. Sulfur has also been observed to solidify on the inner surfaces of the pressure control valve leading to blockage of flow.
Another undesirable affect of large amounts of elemental sulfur in a wet oxidation unit is that the acid formed in even a partial oxidation must be diluted with water to prevent corrosion of the wet oxidation unit. The dilution water increases pipe sizes and heat exchanger requirements and thus increases the cost of the wet oxidation system.
The formation of elemental sulfur from thiosulfate in acidic solutions at elevated temperatures is shown in a paper by Tadaaki Mizoguchi, Yoshiaki Takei and Taijiro Okabe entitled Disproportionation of Thiosulfate, Trithionate, Tetrathionate and Sulfite Under Acidic Conditions, Bulletin of the Chemical Soc. of Japan, Vol. 49 (1), pp. 70-75 (1976).